The use of aromatic onium salts such as diaryliodonium and triarylsulfonium salts as photoacid generators in photolithography and as photoinitiators for cationic photopolymerizations in coating, printing ink and adhesive applications is well documented. The ability to conduct both types of these applications using light with a wide range of wavelengths is an especially challenging undertaking. This is particularly the case in the field of imaging science in which a wide variety of light sources with different emission wavelengths are employed. The next generation of microlithographic photoresists will require the use of shorter UV wavelengths (198 and 157 mu) to produce an even smaller and higher density packing of discrete electronic devices. Most onium salts are intrinsically photoactive in the short and middle wavelength region of the UV spectrum and hence, no photosensitizer is required.
Present microelectronic photoimaging applications employ onium salts for deep UV (I-line, 365 nm) photolithography, and since most onium salts do not absorb at this wavelength, photosensitizers are commonly employed. Further, many current and future imaging processes are being designed using light sources such as lasers and light emitting diodes (LEDs) that deliver radiation in the long wavelength UV and visible regions where onium salts are either poorly responsive or completely inactive. Examples of such imaging processes include stereolithography, rapid prepress prototyping, electronic transmission and printing of images, circuit board imaging, patterning of TV screens and LCD displays and photopolymer printing plates.
There are several additional applications such as dental fillings and restoratives that also make use of long wavelength UV-vis (430-490 nM) radiation. One effective method for spectrally broadening the sensitivity of onium salts photoinitiators at long wavelengths is through the use of photosensitizers. Polynuclear aromatic hydrocarbons such as anthracene, pyrene, and perylene have the requisite long wavelength absorption characteristics and also undergo efficient photoinduced electron-transfer photosensitization with onium salt photoinitiators.
Although polynuclear aromatic hydrocarbons are the most efficient known examples of electron-transfer photosensitizers for onium salts, they have several serious drawbacks that limit their use. For example, they are generally expensive, toxic, and poorly soluble in most reactive monomers and polymer systems. It has been shown that it is possible to prepare derivatives of these compounds that address these latter two issues. However, such derivatives remain prohibitively expensive for general-purpose uses. As a result, a need exists for long wavelength-active photosensitizers that overcome at least one of the aforementioned deficiencies.